How do I unlock all tracks?
The intended in-game way is to play story mode and beat all challenges. If however you want to unlock everything without playing story mode, you can also cheat by editing a config file. Open the folder mentioned above in question "Where does STK store the user config file". From there, open the folder "0.8.2" then open the file "players.xml". Replace all occurrences of "none" with "hard" (or "easy" or "medium", indicating the highest level at which you have solved a challenge).
The cheat is no longer true for 1.0.0. Please update this. I believe there is a config variable somehere that unlock everything without overwriting your Story Mode progress.
EDIT: This question is also outdated: "Where is the configuration stored?"